The thief palms the stamp while it’s being passed around and then arranges to be the one to open the door when the police arrive. While letting them in, he hides the stamp somewhere just outside the room that is searched and then retrieves it later.
Was the stamp hidden outside the room?
Lemmie guess, he licked it, dropped it on the floor and steped on it so it’d stick to the sole of his shoe, then walked off with it even after bieng throughly searched?
Was the stamp in the room at the time of the search?
Does it matter which actual rare stamp it is? Inverted Jenny, etc.
Water, that question was just answered
Did the room have any open windows or other openings?
Was the stamp smuggled underneath the door?
Was the stamp passed through a window?
Did a guest intentionally steal the stamp?
Did the guest lick or moisten the stamp?
Did the thief attach the stamp to the owner when he left the room?
Destroy the stamp (drop it in the fireplace,
swallow it, whatever), then sell a counterfeit on EBay.
Is it at all relevant how the doors locked?
Was there a keyhole in the doors?
Hey, phouka, inquiring minds want to know.
The thief palms the stamp while it’s being passed around and then arranges to be the one to open the door when the police arrive. While letting them in, he hides the stamp somewhere just outside the room that is searched and then retrieves it later.
No.
Was the stamp hidden outside the room?
No.
Lemmie guess, he licked it, dropped it on the floor and steped on it so it’d stick to the sole of his shoe, then walked off with it even after bieng throughly searched?
No, the police are smarter than that.
Was the stamp in the room at the time of the search?
No.
Does it matter which actual rare stamp it is? Inverted Jenny, etc.
No, only that it was unique, not rare.
You know, this is a terrific way to pad one’s posting count. 
Was anyone bragging about the size of the stamp in a phone booth?
New and Improved
Enright3
Yep. That’s it.
ROFL
Was there a time after the stamp was displayed but before the meeting was over?
Did the stamp pass through the doors after it was brought out for display?
“During the meeting, the host brings out a unique and extremely valuable stamp and allows it to be passed amongst the guests. Before he does so, however, he locks the doors to the study so that no one can leave.”
Can I assume that they were actually IN the study at the time?
seriously? No fair.
OK, something’s amiss here.
The stamp wasn’t hidden outside while room was being searched, and it wasn’t in the room while it was being searched.
So the stamp was outside the room, but wasn’t hidden?
New and Improved
Enright3